Method and apparatus for S.I.P./H.323 interworking

ABSTRACT

An interworking function (IWF) for a first and second protocol based network, for example, an H.323 protocol based network and an SIP protocol based network comprises an interworking gateway server including a state machine for defining each call processing state and a translation table for use in translating addresses formatted in each protocol. A method of interworking for use in interworking between said first protocol based network and said second protocol based network comprises the steps of receiving at said interworking gateway server serving said first and second protocol based networks a request from an endpoint in the first or second protocol based networks, establishing a state machine in memory whereby, for each state of said state machine, a message associated with that state is categorized as one of a triggering message, a non-triggering message and an error message, establishing a translation table in said memory whereby an address formatted in said first protocol has a one-for-one correspondence with an address formatted in said second protocol, processing said request in accordance with said translation table and said state machine and permitting communication between said first and second endpoints utilizing a realtime transport protocol. In the event media is terminated at said interworking gateway server, the interworking gateway server, in one embodiment, comprises a media switching fabric for switching media terminated at the gateway to an addressed endpoint capable of receiving it.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 09/825,304, filedApr. 4, 2001, and, like the parent application, claims priority to U.S.Provisional Application 60/195,937, filed Apr. 10, 2000, both of whichare incorporated herein by reference in their entirety.

BACKGROUND

1. Technical Field

The present invention relates to the field of telecommunications systemsin which more than one protocol is being followed and, moreparticularly, to such a method and apparatus for use in suchtelecommunication systems supporting interworking between the Internettelephone protocol: H.323—Packet-Based Multimedia Communications Systemsand the Session Initiation Protocol (SIP).

2. Related Art

The International Telecommunications Union (ITU) is an organization ofthe United Nations promoting the uniform provision of telecommunicationsservices throughout the world. In furtherance of this goal and inparticular, the ITU-T section issued H.323 version 1 in 1996 to describea protocol for Internet protocol telephony applications. Meanwhile, aSession Initiation Protocol (SIP), different from H.323, has beendeveloped since 1999 via the Internet Engineering Task Force (IETF) toalso support Internet protocol telephony applications but arising from adifferent perspective.

In U.S. patent application Ser. Nos. 09/642,142, 09/642,279 and09/642,298, filed Aug. 18, 2000, by Radhika R. Roy and incorporatedherein by reference as to their entire contents, there is shown in theFigures and described a real-time mobility protocol, architecture andintelligent signaling scheme for computer-readable medium for real-timemobile multimedia communications and conferencing over packet-basednetworks. In those applications, the concept of interworking between anH.323 protocol-based network and a Session Initiation Protocol(SIP)-based network is introduced but not described in any detail. InUnited States patent application Ser. No. 09/801,914, filed Mar. 9,2001, by Radhika R. Roy, entitled “H.323 Back-end Services for MobilityManagement” and incorporated herein by reference as to its entirecontents, there are described back-end services supporting mobilitymanagement.

Referring to FIG. 1, there is provided a high level comparison of H.323and SIP showing some of the similarities and many of the differencesbetween the two protocols. The differences result in a problem indefining how interworking, although necessary, results in difficultiesto resolve. For example, not shown in FIG. 1, it may be suggested thatSIP, as it has developed from a different perspective, relates more tothe provision of public and free services while H.323 has evolved fromthe private network side and interests in financial profit. Perhaps,such a comment is not perfectly descriptive, but it may be seen fromFIG. 1, that H.323, for example, is better suitable for exchange, foradmission control, for policy control, for reservation of resources,encourages version compatibility and is binary-based. SIP, on the otherhand, is based on text and is less complex a protocol than H.323.Consequently, there is a need, for example, to translate an address thatmay be in one protocol to an address based in the other protocol.Command codes and messages, too, may not have a one-for-onecorrespondence between one protocol and another. Nevertheless, a userutilizing an H.323 IP telephony device trying to communicate withanother user utilizing an SIP-based IP telephony device or vice versawill have a need to communicate by first requesting service and thenachieving a data connection to the addressed party in as efficient amanner as possible. The requested service may comprise multimedia datatransfers alone or as a supplement to a conventional IP telephony voiceconnection.

Singh et al., in their document “Interworking Between SIP/SDP and H.323”lodged Jan. 10, 2000 with the Internet Engineering Task Force andincorporated herein by reference as to its entire contents, describe afundamental call scenario including user registration and addressresolution and call establishment. They also describe address conversionin some detail whereby, for example, text-based addresses such askns10@columbia.edu are converted to 128.59.19.194 and the like. Also, analgorithm for finding an intersection in capability sets for handlingmedia in terminals and users agents is described. Handling of Q.931 andH.245 messages is also described, and a detailed description of aninterworking gateway behavior is provided. The document which may notcomprise prior art to the present invention fails to describe, forexample, how to classify messages for developing a state machine at thegateway or providing a media switching fabric for any media terminatedat the gateway.

Consequently, there remains a need in the art to provide an interworkingfunction including a state machine for interworking between twodissimilar IP telephony protocols so that users of each protocol will beable to communicate with one another.

SUMMARY OF THE INVENTION

The present invention relates to defining requirements for an SIP-H.323Interworking function (IWF) and provides a conceptual basis for a methodand apparatus for accomplishing such interworking. The IWF is afunctional entity that typically is comprised of one or more networkservers accessible by either a user in an SIP network or an H.323network for receiving service requests and processing such requests toachieve a working connection between terminal devices based on thedifferent protocols without the devices themselves having to be modifiedfor compliance with the other protocol. The one or more servers thuscomprise a gateway converging the ITU-T H.323 and IETF SIP.

A number of definitions will be used consistently herein as follows:

An H.323 gatekeeper (GK) is an optional component in an H.323 network.If it is present, it must perform the functions of address translation,bandwidth control, admission control and zone management.

An InterWorking Function (IWF) according to the present inventioncomprises a gateway server facilitating an interworking between an H.323and a SIP protocol based system.

An SIP Server can be either an SIP proxy or an SIP redirect server asdefined by the SIP protocol.

An endpoint is an entity from which media, as defined below, originatesor finally terminates. This endpoint can either be an H.323 terminal oran SIP user agent.

A media switching fabric (MSF) will be a logical entity present in theIWF, according to the present invention which will perform the task ofswitching of media (e.g., audio, video, and/or data) traffic over thereal-time transport protocol (RTP) from one logical port to other.

A method of interworking via a state machine for use in interworkingbetween an H.323-based network and a session initiation protocol(SIP)-based network comprises the steps of receiving signaling messagesat an interworking server including a data processor and a memory forstoring the state machine that requires translation of signalingmessages between H.323 and SIP. For example, the IWF server will set upa request from an H.323 endpoint, transmitting a corresponding invitemessage to an addressed SIP endpoint, receiving a ringing responsemessage from the SIP endpoint, transmitting a corresponding alertmessage to the H.323 endpoint, receiving an OK message from the SIPendpoint, transmitting a connect message to the H.323 endpoint,negotiating said connect message utilizing an H.245 protocol andtransmitting an ACK message to the SIP endpoint and communicatingbetween the H.323 endpoint and the SIP endpoint utilizing realtimetransport protocol (RTP) using the state machine as a guide. For eachstate, messages and events are characterized as triggering,non-triggering or error. However, an IWF may also include mediaswitching function (IWF), if needed, to switch media (e.g., audio,video, and/or data) traffic terminated at IWF and carried over RTPbetween the H.323 and the SIP side.

A method of interworking via a state machine for use in interworkingbetween a session initiation protocol (SIP)-based network and anH.323-based network comprises the steps of receiving at an interworkingserver an invite message from an SIP endpoint and transmitting acorresponding setup message to an addressed H.323 endpoint, receiving analerting message from the H.323 endpoint and transmitting acorresponding ringing message to the SIP endpoint, receiving a connectrequest message and an H.245 protocol message and transmitting an OKmessage to the SIP endpoint, receiving an ACK message and communicatingbetween the SIP endpoint and the H.323 endpoint utilizing realtimetransport protocol.

Other features of a state machine including translation of SIP and H.323endpoint addresses will become clear from reading the following detaileddescription of the invention when read in conjunction with the drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview drawing providing a high-level comparison betweenthe H.323 protocol and the SIP protocol.

FIG. 2 provides an overall architectural view of a simple end-to-endconnection of an H.323 and an SIP Endpoint (EP).

FIG. 3 shows the architectural overview of FIG. 2 but includes an H.323Gatekeeper function which is optionally available.

FIG. 4 shows the architectural overview of FIG. 2 but includes an SIPserver.

FIG. 5 shows the architectural overview of FIG. 2 but includes both anH.323 Gatekeeper function and an SIP server which may be available.

FIG. 6 is a communication processing chart which shows the steps thatmay be followed in establishing a simple communications session betweenan H.323 Endpoint and an SIP Endpoint.

FIG. 7 is a communication processing chart which shows the steps thatmay be followed in establishing a simple communications session betweenan SIP Endpoint and an H.323 Endpoint.

DETAILED DESCRIPTION OF THE INVENTION

The functionality within an interworking function (IWF) between a firstand a different protocol will now be described with reference to thefigures in which reference numerals will be used to consistentlydescribe similar elements. The differences between SIP and H.323 havealready been described with reference to FIG. 1.

Referring to FIG. 2, an interworking function which may mostconveniently comprise a gateway server is shown as IWF 100 comprising amedia switching fabric 110 in the event variable media is involved, forexample, video, graphics, data, encoded voice, facsimile and the like,that may require switching. The functional requirements of an SIP-H.323interworking function 100, in particular, necessitate close scrutiny ofcommunications processing in each protocol and accommodating anydifferences, for example, in addressing and command coding. To this end,the gateway server of IWF 100 may further comprise a data processor 105and memory 115 for, for example, call processing programs includingaddress and command translation programs and databases for storingaddress translation data. In particular, as will be further describedherein, a state machine is provided in gateway memory whereby, for eachstate of call processing, an event or message is defined as triggering,non-triggering or an error message and “triggering” further may requirethe definition of an action and the next state of the state machine.External access for IWF 100 should be assumed in FIGS. 2-5 for queryingexternal databases and servers (not shown) in order to accomplishinterworking.

Referring to FIGS. 3-5, there are shown variations on a simplifiedarchitecture of FIG. 2. FIG. 2, as already described, shows simply anH.323 Endpoint 125 being coupled to an SIP Endpoint 150 via IWF 100where the IWF functionality may be considered to be included eitherwithin H.323 network 160 or SIP network 170 or, as not shown, astand-alone functionality while FIGS. 3-5 assume one or more of an H.323gatekeeper 200 or SIP server 300 are present.

It is clear that an interworking function (IWF) for two differentprotocols such as H.323 and SIP can be architectured in various ways. Anexemplary architecture may include the coexistence of H.323 Gatekeeper(GK) 200 with IWF 100 (FIG. 3) or SIP servers 300 (FIG. 4) with IWF 100or both H.323 GK 200 and SIP server 300 may be present with IWF 100(FIG. 5). The location of the H.323 GK 200 and/or SIP server 300 inconjunction with the IWF 100 is a matter of implementation and not aprotocol issue. There will be no assumptions made for the optionalelements and components present in either H.323 or SIP networks. Thesolution provided here will work for a minimum configuration requiredfor both the protocols. Below will be described recommendations forother configurations, which may include other optional components.

For instance, H.323 Gatekeeper 200 is not a mandatory component of anH.323 network 160. So, there will be no assumptions made for the basicinterworking which involves H.323 Gatekeeper 200. For example, in FIG.3, H.323 Gatekeeper function 200 may be either co-located with IWF 200or may exist separately in an H.323 zone in which the H.323 entity 125,which may be mobile, resides.

The introduction of IWF redundancy in the network may be desirable. Forexample, it may be desirable to have plural IWF 100 in a geographicregion in the event of failure of one or another or geographicallydispersed IWF may cover for one another in the event of a failure of oneor the other.

In view of the requirements of providing such an IWF gatewayfunctionality, an IWF 100, in whatever architecture is available amongthe architectures of FIGS. 2-5, the IWF 100 is assumed to contain thefollowing functions: a) Call sequence mapping (for example,message/command mapping); b) Address translation and resolution (SIP toH.323 and vice versa); c) Terminal Capability transactions (for example,endpoint terminal capabilities to handle various media); d) Opening andclosing of media channels; e) Mapping of media codecs for H.323 and SIPnetwork; f) Resource reservation and release; g) Ability to provide thestate of resources (resource busy/idle state tables and the like); h)Call state machine (what the IWF 100 should do when it finds itself in agiven state); and i) Out of band signal processing, among others.

It is preferable that there be no processing on the media data at IWF100. It is assumed that both H.323 network 160 and SIP network 170 usethe realtime transport protocol (RTP) as a transport for carrying mediabetween endpoints. In most of the cases, RTP will be utilized directlybetween the endpoints 125, 150 as will be discussed in connection withFIGS. 6 and 7 and steps 690 and 790 respectively. Even if the media fromone or the other endpoint 125, 150 is terminated at IWF 100 in a specialscenario, the assumption is made that this media will be simply switchedto another selected endpoint by a media switching fabric 110 present inthe IWF 100.

The inclusion of Network Management for IWF 100 may be left to the H.323network and/or SIP network management functions separately present withorganizational control delegated to the network first requiring a givennetwork management operation.

Pre-Call Requirements include determining the location of an entitydesiring service among other requirements. For example, in Mr. Roy'sprior pending U.S. patent application Ser. No. 09/801,914, entitled“H.323 Back-end Services for Mobility Management,” filed Mar. 9, 2001,back-end services are described for an H.323 network.

The IWF function 100 shall have a table of reference in its memory forlook up to resolve the corresponding H.323 and SIP addresses to IPaddresses. This look-up table can either be accomplished by using thecapabilities of H.323 Gatekeeper 200 and/or SIP server 300. In oneembodiment, the memory table may be stored locally at IWF 100 andperiodically updated by H.323 network 160 and SIP network 170. SinceH.323 Gatekeeper 200 and SIP server 300 are not mandatory components ofthe H.323 and SIP systems of FIGS. 2-5 respectively, the IWF function100 may keep the table of information for address resolution withinitself, which, in another embodiment, can be updated by the IWF function100 by querying the H.323 Gatekeeper 200, SIP server 300 or any otherdatabase external to the IWF 100.

Registration with H.323 Gatekeeper 200 by H.323 entity 125 is discussedin Mr. Roy's earlier applications. Registration is performed for an SIPnetwork 170 also, for example, to give the information about SIP sideextensions of IWF 100 to H.323 Gatekeeper 200 if it is present in thenetwork. This information will be used by H.323 Gatekeeper 200 to directa call whose destination is in the SIP network 170. The registrationinformation may be updated at any time to the H.323 Gatekeeper 200, forexample, periodically by SIP network 170.

IWF 100 can preferably register with one H.323 Gatekeeper 200 only. Theability to register with multiple H.323 GK gatekeepers by one IWF 100may be desirable in some situations, for example, when the gatekeepersserve multiple domains or in the redundancy situation alluded to above.

Registration with SIP server 300 also must be accomplished for the H.323side of the architecture. This registration is done to give informationabout H.323 side extensions of the IWF 100 to the SIP server 300 if itis present in the SIP network 170. This information will be used by theSIP server 300 to direct a call whose destination is in H.323 network160. The way in which IWF 100 gets the information of H.323 sideextensions may be periodic update by the H.323 gatekeeper if availableor may be initiated by the IWF 100 or by the H.323 Gatekeeper 200, ifpresent. IWF 100 may preferably register with one SIP server 300 only.However, IWF 100 may register with more than one SIP server 300, forexample, when the SIP server 300 serves different endpoints or whenredundancy is provided.

Resource Management is the management and allocation of, for example,ports, addresses, and bandwidth for conveying data packets. Resourcesfor a call also include the memory requirements, (in addition to theprocessing time slot), logical ports and other call-related data withinthe IWF 100. Resources in the IWF 100 are to be managed with respect toresource reservation and resource control functions of the gatewayserver. This resource reservation is done for both signaling and mediaswitching fabric 110 on a per-call basis depending on call requirementswhich may involve multiple media and multiple endpoint signaling.

Resource Allocation and Reservation is performed as well by IWF 100. TheIWF shall: support reservation of logical ports for signaling and mediaswitching fabric 110 for use by a particular call and support theirsubsequent release (which may be implicit or explicit); allow release ina single exchange of message (minimizing any negotiation) of allresources associated with a particular call; support release ofresources if IWF 100 detects that the call is no longer active; andsupport the reservation and release of resources for opening, reopening,changing and closing of media channels during the call.

The IWF may support the reservation by priority based on the order ofcapability descriptors and support the reporting of resource reservationand connection completion.

Resource control is another important function of IWF 100. The IWF 100shall support the pre-reservations for a particular call. Thesereservations can be made before a call appears at IWF 100. The IWF 100shall also support the restrictions that can be imposed on a particularendpoint for the use of resources; support the pre-reservation ofresources for a particular endpoint; support the reporting out ofresources; support the denial of additional resources required during acall for opening, reopening, closing and changing of sessions, andprovide support for forced release of the resources associated with acall.

There are also what may be called General Interworking Requirements ofIWF 100. The IWF gateway 100 shall use, for example, the H.323 Version2.0 and SIP Version 2.0 and be forward version compatible. The IWFgateway 100 should handle all mandatory features of H.323 Version 2 aswell as SIP Version 2.0 and should also provide backward compatibilityfor earlier versions. In the future, an augmentation of the IWFcapabilities will also support for interworking with other versions, forexample versions 2, 3, 4, 5, etc. of H.323 and SIP.

The IWF 100 will provide for the seamless interworking of the twoprotocols. By seamless is intended a smooth interworking, invisible tothe user of either the H.323 terminal device or SIP terminal device atendpoints 125 and 150 respectively. The functioning of IWF 100 shouldnot involve any modification to the H.323 and SIP protocols but mayinvolve specific profiles of these protocols.

There are a number of objectives in defining performance requirementsfor IWF 100. These include but are not limited to the following: 1)Minimizing the message exchange between IWF 100 and either an H.323network 160 or SIP network 170 apparatus; 2) Establishing apredetermined recommended maximum processing delay at IWF 100 (the delayfigure will be dependent on the expected round-trip message delays andthe amount of message exchange); 3) Establishing guidelines for peakcalling time (busy periods for various media); and 4) Establishingdefault settings so that transactions will only be for a change todefault parameters or for setting non-default parameters.

IWF 100 should support certain basic call requirements such asresolution of different addresses and address formats for H.323 and SIPendpoints. In this regard, the IWF 100 shall support all the addressingschemes of both the H.323 and SIP protocols. The IWF 100, for example,shall register itself to each of an H.323 Gatekeeper 200 and SIP server300 either directly or via some indirect method as appropriate, if theyare present in the respective networks 160 and 170. For example, asdescribed in the previous location, an H.323 Gatekeeper 200 maintainsH.323 mobile entity address, location and identification data or iscapable of obtaining such information from related databases. The IWF100 can facilitate address translation by maintaining a look up tablefor resolving the addresses because, for example, address resolution isrequired when an H.323 endpoint requests connection to an SIP endpointor vice versa. The look-up table of IWF 100 may be updated periodicallyor upon query by the IWF 100 from the H.323 GK 200 and SIP server 300.The IWF 100 may alternatively use Lightweight Directory Access Protocol(LDAP) or X.500—Directory Protocol (ITU-T) for keeping the addressresolution information or use Domain Name System (DNS) for addressresolution.

When a call occurs utilizing an H.323 GK 200 per FIG. 3, the IWF 100shall resolve addresses with the help of H.323 GK 200 when it is presentin the network 160. Moreover, the IWF 100 shall register itself toforward the SIP extensions supported on the SIP network 170 side of theIWF 100. At this time, and until redundancy is supported, it shall notbe necessary for IWF 100 to register with two different H.323Gatekeepers 200. Most conveniently, the IWF 100 may update any newlyadded SIP extensions of SIP network 170 to H.323 Gatekeeper 200.

When a call occurs utilizing an SIP server 300 per FIG. 4, the IWF 100shall resolve addresses with the help of SIP server 300 if it is presentin the network 170. Moreover, the IWF 100 shall register itself with SIPserver 300 to forward any H.323 extensions supported on the H.323network 160 side of IWF 100. At this time and until redundancy issupported and in order to reach all SIP endpoints 150, the IWF 100 mayregister with many SIP servers 300 and update any newly added H.323extensions to SIP server(s) 300.

When a call occurs utilizing both an H.323 GK 200 and one or more SIPservers 300 as per FIG. 5, all the requirements defined above when onlyone or the other of an H.323 gatekeeper 200 (FIG. 3) or one or more SIPservers 300 (FIG. 4) will be met for this case as well.

Due to inherent capability differences between apparatus of an H.323network 160 and SIP network 170, IWF 100 must support capabilitynegotiation.

Consequently, the IWF 100 shall not make any assumptions for thecapabilities of either an SIP user agent or an H.323 terminal atendpoints 125, 150. However, IWF 100 may indicate a default capabilityof an H.323 terminal or SIP user agent even before doing capabilityexchange with H.323 (for example, using H.245—Control Protocol formultimedia communication) and SIP (for example, using SessionDescription Protocol [SDP]). This default capability includes themandatory capability requirements as defined by the respectiveprotocols. For example, G.711 (ITU-T) coding is mandatory for higherbandwidth networks 160 within H.323. The IWF 100 shall pass on all thecapability descriptors of H.323 and SDP from SIP in the maximum possibleway to each other. The algorithm for finding out the maximum mapping ofcapability descriptors with the corresponding SDP is left for furtherdiscussion. Moreover, the IWF 100 shall provide mapping for commonaudio/video formats supported in H.323 with the Realtime TransportProtocol with Audio/Video Profile (RTP/AVP) formats.

To know the capabilities of an SIP entity, the IWF 100 may use anOPTIONS message and may use SDP for capability negotiations on the SIPside, while H.245 may be used to know the capabilities as well as forcapability negotiation on the H.323 side. However, the capabilitydiscovery and negotiations by the IWF 100 can be done in atransport-independent way that can be applicable for any transportnetwork such as Internet Protocol (IP), Asynchronous Transfer Mode(ATM), and other networks. Moreover, the IWF 100 may supportre-negotiation of codec and other resource assignments, if needed.

The IWF 100 also supports the opening and closing of logical channels.Toward this objective, the IWF 100 shall open (and close) the channelsbetween the endpoints 125, 150 only wherever possible. If it is notpossible to do so between the endpoints 125, 150, then a given channelcan be opened at the media switching fabric 110 of IWF 100. Moreover,the IWF 100 shall support unidirectional, symmetric bi-directional, andasymmetric bi-directional opening of channels so endpoints may be addedor dropped, for example, for unidirectional traffic or conferencingapplications. The IWF 100 may respond to the mode request and/or to therequest for reopening and changing an existing logical channel andsupport the flow control of H.323.

The IWF 100 also supports and handles media transmission and reception.By media is intended to include at least the information content of aninformation delivery from one endpoint to another. Toward thisobjective, the IWF 100 shall not process RTP data going in and out frommedia switching fabric 110, but rather shall leave any RTP dataprocessing to respective networks 160 and 170. As will be seen from thediscussion below of FIGS. 6 and 7, RTP is relied upon forendpoint-to-endpoint media transfer. Moreover, the IWF 100 may collectstatistics on media flow for a particular call between endpoints 125,150. The parameters for collection of statistics should permit thedefinition of peak calling times for various media. The collection andmaintenance of statistical data for various media can provide flowstatistics so that the combined networks 160 and 170 may maintain atleast a minimum quality of service (QOS).

The IWF 100 also must support fast connect procedures. For example, theIWF 100 shall support the fastStart element.

Also, the IWF 100 must support H.245 tunneling, for example for moreefficient call set-up. To this end, the IWF 100 shall support the H.245tunneling in a Setup message.

Moreover, an IWF 100 must support pregranted Admission Request (ARQ).The IWF 100 shall support the pregranted ARQ by accomplishing addressresolution from the H.323 GK 200 using location request (LRQ) andlocation confirm (LCF) message exchange.

Also, the IWF 100 shall support overlapped sending of digits, forexample, the overlapped sending of dialed digits.

The IWF 100 shall support early versions of H.245, if at all possible.

Now data transport will be discussed in some detail. Firstly, certainassumptions will be made for underlying networks 160 and 170. Theunderlying network should support both the TCP and UDP, i.e. bothreliable and non-reliable delivery of messages is supported. Thenetworks 160 and 170 of a given H.323 and SIP system can begeographically anywhere. There are no assumptions for the closeness ofthese networks to one another. The underlying network is not assuringlevels of quality of service (QOS) beyond a minimum. There is nopriority of signaling messages over other messages.

The transport requirements for interworking include an assumption thatboth H.323 network 160 and SIP network 170 use RTP for carrying media.If this is not the case, then a media gateway will be required, whichmay not be desirable, between endpoints. The transport should supportfor large fan-out to multiple endpoints.

The IWF 100 must support mapping between SIP and H.323 messages. Ingeneral, a clearer mapping between SIP and H.323 messages shall beprovided which reflects similar command meanings in call processsequence. The call message sequence shall be maintained in both thedirections of transmission of media. The IWF 100 shall not make anydecision on its own related to basic functionality of a call, like callsetup and call teardown, etc. Any H.323 or SIP messages, which do nothave a match on the other side, should be terminated on the IWF 100, andIWF 100 should take the necessary action on resolving them. In case theIWF 100 is required to generate a message on its own toward the H.323 orthe SIP side, IWF 100 should use pre-configured default values for theparameters.

The information elements of the respective messages of the H.323 and SIPprotocols are to be converted as follows: a) The contents of connectionspecific information elements (such as Call Reference Value on H.323)shall be converted to respective information as required by SIP or SDPsuch as session identification (ID), call leg and Call-ID; b)Information elements that are not in use on the H.323 network 160 sideof IWF 100 shall be generated by the IWF 100 as required by the SIPprotocol and vice versa; c) The SIP data fields are converted into thecorresponding Abstract Syntax Notation One (ASN.1) user-user informationelement structure. The user-user information element structure shall begenerated according to the specification in Recommendation H.225.0 formiddleware and H.245 (for opening and closing logical channels).

Now call signaling registration and admission (per H.225.0) and SIP callsignaling will be discussed. The IWF 100 shall conform to the callsignaling procedures recommended for the SIP network 170 sideindependent from the H.323 network 160 side, and the IWF 100 shall alsoconform to the call signaling procedures recommended for the H.323network 160 side independent from the SIP side.

The IWF 100 shall terminate the Q.931 Call Signaling Channel between anH.323 endpoint 125 or H.323 Gatekeeper 200 (in case of H.323 GK routedsignaling) and the IWF 100 on the one hand and the call signaling (ifany) between the IWF 100 and the SIP endpoint 150 on the other SIPnetwork 170 side. The IWF 100 shall terminate the Registration,Admissions and Status (RAS) Channel between an H.323 Gatekeeper 200 (ifany) and IWF 100.

The IWF 100 supports messages for supplementary services (FACILlTY,NOTIFY, and the INFORMATION messages) on the H.323 network side byprocessing such messages at the IWF 100, but only if the associatedservice is supported.

The IWF 100 should support H.323 Call Control (H.245) and SIP CallControl (SDP) and, to that end, IWF 100 should try to map the H.245 andSDP to the maximum extent.

The IWF 100 shall also support H.323 audio/video codec to SIP mediaformats. To this end, the IWF 100 should provide invisible support forall audio/video algorithms supported by either ITU or IANA and mayhandle dynamic payload types.

The call sequence should be maintained in such a way on both sides ofIWF 100 so that neither H.323 terminal nor SIP UA is aware of thepresence or existence of an IWF 100. The IWF 100 should provide seamlessinterworking between the call flows of the two protocols. The IWF 100should limit any modifications to the normal call flows of eitherprotocol. The messages and parameters which do not have direct mappingon the other side are to be generated by the IWF 100 with defaultparameters in most of the cases. In brief, the H.323 endpoint 125 shouldnot be aware of the fact that it is calling an SIP endpoint 150 and viceversa.

The definition of a state machine for IWF 100 will now be described. Astate machine for IWF 100 will follow the following general guidelines:a) Unexpected messages in a particular state are treated as “Error”messages; b) All messages which do not change the state are treated as“Non triggering or Informational” messages; c) All messages which expecta change in state are treated as “Triggering” messages.

To this end, for each state, for example, busy and idle states amongother states defined in the state machine, there should be guidelinesthat classify all possible messages into the above three categories.Apart from this, it is required to specify the processing i.e. action tobe taken in the state machine on the contents of the message. Triggeringmessages expecting a change in state will result in showing the nextstate in the state machine table.

This classification of messages will result in a table provided below asan illustration. In the table, note that for a triggering message/eventsuch as a registration message, an action “Add registration information”is defined and the next state “WaitForSetup is defined while for, forexample, a non-triggering message such as a Q.931 message, thisinformation is not shown. Similarly, an “error” message such as an H.245message has no action or next state defined. State: Idle PossibleMessage Messages Category Action Next state All RAS Msg. Triggering AddReg.Info. WaitForSetup All Q.931 Msg. Non Triggering All H.245 Msg.Error All Msg. From Triggering SIP side

The IWF 100 should additional support security requirements which may bestricter on the H.323 side than on the SIP side.

In call sequencing, there are assumptions that can be made for theendpoints 125, 150: 1) All endpoints trying to use IWF 100 areauthorized with respective H.323 Gatekeeper 200 and SIP server 300 ifone or the other or both are present in their respective networks 160and 170; 2) All endpoints trying to make a call using IWF 100 arerespectively admitted to do so from H.323 Gatekeeper 200 and SIP server300 if present in their respective networks 160, 170.

The IWF 100 shall be required to provide procedures for preventingdenial of service security attacks and maintaining persistence data forauthorized endpoints for future verifications.

The IWF ought to support simple call supplementary services like callforwarding, call hold and call transfer, conferencing, session change(re-invite, mode request), security: Authentication, Authorization andprivacy, quality of service (QOS) signaling, network management andredundancy.

Now, we will discuss some examples of call scenarios that will showprimarily the input and output signaling messages of the IWF 100 forinterworking between SIP and H.323 with reference to FIGS. 6 and 7. Theimportant point is that the IWF 100 will perform the translation betweenthe signaling messages of SIP and H.323. We will show what should be theoutput signaling message of the IWF 100 for a given input signalingmessage in the IWF 100.

In performing mapping of addresses and commands, the IWF 100 may have toface the following situations: a. It may so appear that there can beone-to-one mapping between the signaling messages and the IWF 100 willperform the translation accordingly; b. All parameters used in eachsignaling message one side may not match exactly to the correspondingsignaling message of the other side. In this situation, somemanipulations need to be done by the IWF 100 so that an agreed uponstandard can be created based on common understanding although allparameters do not exactly match; c. For a given signaling message of agiven protocol, there may not be a corresponding signaling message ofthe other protocol that may appear to be equivalent. The IWF 100 willcreate and maintain a mapping between the signaling messages or generateerror messages based on common understanding of an agreed upon standard.These items a, b, and c as stated above are critical to creating aninteroperability standard between H.323 and SIP. Some issues in thoseareas have already been addressed by Singh et al. in their documentdescribing interworking between SIP/SDP and H.323: Singh/Schulzrinne,“Interworking Between SIP/SDP (Session Descriptive Protocol) and H.323”,singh-sip-h323-00.txt, IETF, January 2000. However, we have addressedabove and below, for example, the design of a state machine, theconfigurations for call scenarios and the input-output messages of theIWF 100 among other topics not addressed by Singh et al. that arerequired to provide interoperability between SIP and H.323.

The different call scenarios for the configurations of FIGS. 1-5 that weforesee are: a) Simple Call from H.323 terminal to SIP terminal; b) Callfrom H.323 terminal to SIP terminal using H.245 tunneling; c) Call fromH.323 terminal to SIP terminal using early H.245; d) Call from H.323terminal to SIP terminal using fast connect procedure; e) Call fromH.323 terminal to SIP terminal using overlapped sending; f) Call fromH.323 terminal to SIP terminal using pre granted ARQ (for configurationshaving H.323 GK 200); g) Simple call from SIP terminal to H.323terminal; h) Call from SIP terminal to H.323 terminal using H.245tunneling; i) Call from SIP terminal to H.323 terminal using earlyH.245; j) Call from SIP terminal to H.323 terminal using fast connectprocedure; k) Call from SIP terminal to H.323 terminal using overlappedsending; and 1) Call from SIP terminal to H.323 terminal using pregranted ARQ (for configuration having H.323 GK 200).

Referring to FIG. 6, a simple call from an H.323 terminal at endpoint125 to an SIP terminal at endpoint 150 using the configuration of FIG. 1involves the steps of: step 610, the H.323 endpoint 125 generating aSetup message to IWF 100 when IWF 100 must translate and identify theaddressed endpoint 150; step 620, the IWF 100 generating an Invitemessage 620 to SIP endpoint 150; SIP Endpoint 150 in step 630 generatinga 180 Ringing message to IWF 100 whereupon in step 640, IWF 100transmits an Alerting message to H.323 endpoint 125. Also, SIP endpoint150 transmits a 200 OK message to IWF 100 resulting in the IWF 100transmitting a Connect message 660 to H.323 endpoint 125. Step 670represents H.245 tunneling between H.323 endpoint 125 and IWF 100. Oncethe H.245 messaging is completed, IWF 100 sends an ACK message to SIPendpoint 150. Thereafter, the respective endpoints 125 and 150communicate via RTP in step 690.

Referring to FIG. 7, a simple call from SIP terminal at endpoint 150 toan H.323 terminal at endpoint 125 using the configuration of FIG. 2 hasthe following call processing sequence. Initially, SIP endpoint 150transmits an Invite message in step 710 which is received at IWF 100that is responsible for translating and identifying addressed (orcalled) endpoint 125. IWF 100 in step 720 transmits a Setup message toH.323 endpoint 125. Then, H.323 endpoint 125 transmits an Alertingmessage to IWF 100 in step 730. Upon receipt of the alerting message,the IWF 100 generates a 180 Ringing message for transmission to SIPendpoint 150 in step 740. In step 750, H.323 endpoint 125 transmits aConnect message to IWF 100. In step 760, H.323 endpoint 125 and IWF 100negotiate via H.245 tunneling. Once the negotiation is complete, IWF 100transmits a 200 OK message to SIP endpoint 150 which replies with an ACKmessage to IWF 100 in step 780. Thereafter, the respective endpoints 125and 150 communicate via RTP in step 790.

The following references should be deemed incorporated by reference asto any contents deemed necessary to an understanding of the presentinvention: M. Handley, H.Schulzrinne, E. Schooler, and J. Rosenberg,“SIP: Session Initiation Protocol”, RFC 2543, IETF, March 1999; M.Handley and V. Jacobson, “SDP: Session Description Protocol”, RFC 2327,IETF, April 1998; S. Bradner, “Key words for use in RFCs to indicaterequirement levels”, RFC 2119, IETF, March 1997; and “Packet basedmultimedia communication systems”, Recommendation H.323, ITU-T, Geneva,Switzerland, February 1998.

Thus, there has been described a framework for H.323/SIP interworkingwhich meets the objectives sought of a simple, efficient interworkingmechanism. Other features and advantages of the present invention willbe understood with reference to the claims below which should not bedeemed to be limited to the specific disclosure of one embodiment of thepresent invention.

1. A method of interworking via a state machine for use in interworking between an H.323 based network and a session initiation protocol (SIP) based network comprising the steps of: receiving at an interworking server, including a data processor for call processing and a memory for storing said state machine, a set-up request from an H.323 endpoint, transmitting a corresponding invite message to an addressed SIP endpoint, receiving a ringing response message from the SIP endpoint, transmitting a corresponding alert message to the H.323 endpoint, receiving an OK message from the SIP endpoint, transmitting a connect message to the H.323 endpoint, negotiating said connect message utilizing an H.245 protocol, transmitting an ACK message to the SIP endpoint, and communicating between the H.323 endpoint and the SIP endpoint utilizing realtime transport protocol (RTP).
 2. A gateway server for performing the method of claim 1, said interworking server comprising said gateway server.
 3. A gateway server as recited in claim 2 wherein said interworking server further includes a media switching fabric for switching media terminated at the interworking server.
 4. A method of interworking as recited in claim 1 further comprising the step of establishing a state machine table in said memory whereby, for a state of said state machine, a message associated with said state is categorized as one of a triggering message for triggering a predetermined action, a non-triggering message and an unexpected message in said state.
 5. A method of interworking as recited in claim 1 wherein said set-up request receiving step further comprises the step of translating an SIP address to an H.323 address.
 6. A method of interworking as recited in claim 5 wherein said translation step comprises the preliminary step of receiving SIP address data from an SIP server for storage in said memory.
 7. A method of interworking as recited in claim 4, for an idle state, defining registration messages as triggering an addition of registration information, a Q.931 message as non-triggering and an H.245 message as an error message.
 8. A method of interworking via a state machine for use in interworking between a session initiation protocol based network and an H.323 based network comprising the steps of: receiving at an interworking server, including a data processor for call processing and a memory for storing said state machine, an invite message from an SIP endpoint and transmitting a corresponding setup message to an addressed H.323 endpoint, receiving an alerting message from the H.323 endpoint and transmitting a corresponding ringing message to the SIP endpoint, receiving a connect request message and an H.245 protocol message and transmitting an OK message to the SIP endpoint, receiving an ACK message, and communicating between the SIP endpoint and the H.323 endpoint utilizing realtime transport protocol.
 9. A gateway server for performing the method of claim 8, said interworking server comprising said gateway server.
 10. A gateway server as recited in claim 9 wherein said interworking server further includes a media switching fabric for switching media terminated at the interworking server.
 11. A method of interworking as recited in claim 8 further comprising the step of establishing a state machine table in memory whereby, for a state of said state machine, a message associated with said state is categorized as one of a triggering message for a predetermined action, a non-triggering message and an unexpected message in said state.
 12. A method of interworking as recited in claim 8 wherein said set-up request receiving step further comprises the step of translating an H.323 endpoint address to an SIP endpoint address.
 13. A method of interworking as recited in claim 12 wherein said translation step comprises the preliminary step of receiving H.323 endpoint address data from an H.323 gatekeeper.
 14. A method of interworking for use in interworking between a first protocol based network and a second protocol based network comprising the steps of: receiving at an interworking gateway server serving said first and second protocol based networks a request from an endpoint in the first or second protocol based networks, establishing a state machine in memory whereby, for each state of said state machine, a message associated with that state is categorized as one of a triggering message for a predetermined action, a non-triggering message and an error message, establishing a translation table in said memory whereby an address formatted in said first protocol has a one-for-one correspondence with an address formatted in said second protocol, processing said request in accordance with said translation table and said state machine and permitting communication between said first and second endpoints utilizing a reliable transport protocol.
 15. A method as recited in claim 14 further comprising the step of switching any media terminated at said interworking gateway to an addressed endpoint.
 16. An interworking gateway server for performing the method of claim 14, said interworking gateway server comprising said state machine, said translation table and a data processor.
 17. An interworking gateway server as recited in claim 16 further comprising a media switching fabric for switching media terminated at said interworking gateway server.
 18. A method of interworking as recited in claim 14 wherein said first protocol based network is an H.323 protocol based network and said second protocol based network is a SIP protocol based network.
 19. A method of interworking as recited in claim 14 where for an idle state a registration message is a triggering message for an action of adding registration information, a Q.931 message is non-triggering and an H.245 message is an error message.
 20. A method as recited in claim 19 wherein for said idle state, the next state comprises a WaitForSetup state. 